1. Field of the Invention
The present invention generally relates to compositions and methods for treating subterranean formations in order to make them more permeable. Such subterranean formations may, for example, be comprised of silicate minerals or carbonate minerals. Be the subterranean formation minerals as they may, this invention is especially concerned with matrix acidizing and/or fracture acidizing them in order to increase their permeability. Matrix acidizing is conducted below formation-fracturing pressures. Conversely, fracture acidizing is conducted at formation-fracturing pressures. In either case, any resulting increase in permeability of a formation can lead to increased production of a targeted material contained therein. For example, increased permeability in a hydrocarbon-bearing formation can lead to increased petroleum and/or natural gas production. Such increased permeability can also lead to increased production of non-hydrocarbon materials (e.g., carbon dioxide, sulphur, water, helium, etc.) from subterranean formations containing such materials.
Most matrix acidizing operations are aimed at increasing hydrocarbon production by dissolving subterranean formation clogging materials (especially those located near a borehole) and/or by invasion of existing pores and fractures in a subject formation. Any of these operations can be accomplished by pumping treatment fluids (e.g., acidic, aqueous solutions and/or gases) into a subject subterranean formation under pressures and flow rates such that the treatment fluid flows to and around any targeted subterranean formation clogging materials and/or into existing pore spaces and/or into existing fractures in the formation that may be clogged by granular materials. The acid components of such treatment fluids then chemically react with certain minerals contained in the formation clogging materials, pore spaces and clogged fractures. Such matrix acidizing operations also can create so-called “wormhole” systems in a matrix acidized formation. In effect, such wormhole systems are complex, three dimensional arrays of interconnected passageways.
Those skilled in the subterranean treatment arts will appreciate that there are at least four general types of matrix acidizing treatments: (1) wellbore cleanouts, (2) near-wellbore stimulation treatments, (3) intermediate matrix stimulation treatments and (4) extended matrix acidizing treatments. Each calls for use of different treatment techniques according to the distance between a wellbore and a targeted zone in a given subterranean mineral body. Those skilled in these arts also will appreciate that the acid treatment solutions used in each of these four treatment techniques tend to penetrate into subterranean formations for only relatively short distances before they are chemically spent. Indeed, this fact is part of the underlying basis for distinguishing between wellbore cleanouts, near-wellbore stimulation treatments, intermediate matrix stimulation treatments and extended matrix acidizing treatments.
It also should be understood that the selection of acids (and their concentrations) for each of these four treatment methods involves, among other things, further consideration of a given subterranean formation's: (1) mineral composition, (2) structure, (3) permeability, (4) porosity, and (5) physical strength. Other factors which then must be considered in the acid identity (concentration) selection process include, but are by no means limited to: (6) reservoir fluid properties, (7) temperatures, (8) pressures, and (9) any limitations on treatment fluid injection rates. Moreover, the identity and amounts of various additives, e.g., corrosion inhibitors, surfactants, and iron-control agents, friction reduction agents and so on, will vary with changes in the identity of a treatment acid (and its concentration). Cost considerations, ease of mixing, ecological concerns and safety considerations also are important factors in most matrix acidizing operations.
By way of distinction from matrix acidizing operations, fracture acidizing operations are carried out by pumping acidic fluids into subterranean formations at pressures and flow rates high enough to fracture that formation. There are also at least four primary fracture acidizing techniques: (1) fluid-loss control, (2) conductivity enhancement, (3) etched height control and (4) a variety of very specifically tailored fracture treatments. Regardless of the type of fracture acidizing technique being carried out, the acidic components of the high pressure fluids employed generally serve to etch fluid flow channels in newly fractured regions of that formation. The treatment solution volumes needed to carry out most fracturing operations are, however, generally much larger than those required for matrix acidizing operations. Hence, the expense of a fracture treatment solution may become a far greater factor relative to that of a matrix acidizing operation. Such cost of materials considerations also imply extensive design and/or lab work to determine, among other things, the mineral nature of the formation being fractured, identification of the most suitable acids, their optimal concentrations and/or the need for other chemical agents and/or particulate materials in the fracture treatment solution selected.
For example, some fracture treatments call for the use of particulate materials such as silica flour and 100-mesh sand particulates. That is to say that such particulate materials can be used to advantage in some fracture acidizing operations—but not in others—depending on the treatment acid selected. Other granular materials (e.g., graded rock salt, benzoic acid flakes, wax beads, wax buttons and/or oil-soluble resin materials) may have to be employed in other fracture acidizing operation depending on the identify of the acid selected (e.g., HF versus HCL). Selection of any of these particulate materials also implies further consideration of a host of subtle, complex and interrelated factors that very often compete with each other when they are used in the same subterranean treatment solution and/or fracture acidizing technique.